1. Field of the Invention
The present invention relates to a plastic lens used in optical elements such as cameras and optical disc apparatus, a manufacturing method of the same, a production tracing management method for a plastic lens, and an optical pickup device having a plastic lens.
2. Description of the Related Art
Digital cameras and optical disc apparatus have been spread. The optical disc apparatus, for example, have an optical pickup device for reading information stored on an optical storage medium. The optical pickup device has a pickup lens for focusing light from a light source on the optical storage medium.
The pickup lens is generally manufactured by molding glass or plastic such as polyolefin resin or acrylic resin. A plastic pickup lens is produced by injection molding, transfer molding, or other molding techniques using a mold. In an injection molding process, for example, resin is poured into a cavity in the mold through a gate and allows it to cool to solidify, thereby producing a pickup lens.
This kind of plastic lenses are mass-produced to reduce unit price. Thus, a plurality of lenses are simultaneously produced using a mold having a plurality of cavities. For example, eight lenses are produced at a time using a mold having eight cavities. Though the quality of the lens formed in each cavity is preferably the same, it may vary in practice. Further, the quality may vary by mold and molding device.
A technique of marking a plastic lens is disclosed in Japanese Unexamined Patent Application Publication No. 2002-122711, and United States Patent Application Publication No. 2001/0017733, Asakura, for example. To distinguish a lens surface, this technique deposits a coating film that serves as a marking on the peripheral area of one lens surface by printing, coating, or evaporation. Thus, the material of the lens and the marking are different; therefore, this technique further requires a marking process in addition to a lens molding process when producing a lens by injection molding.
Another marking technique for another object is disclosed in Japanese Unexamined Patent Application Publication No. 2000-111709. This technique creates a convexity or concavity on a flange surface of an optical component to indicate a resin injection position. No mention is made to a manufacturing method nor mold used in this technique. Since the convexity or concavity is placed on the upper surface of the flange part, however, the marking is likely to chip away or get contaminated when put on a desk or by handling. This causes to fail to convey marking information properly. Besides, the whole purpose of the marking described in this technique is to align the position of a gate, not to identify a lens nor track the production of the lens during the production process. This is obvious from the fact that the marking is formed only in one place, thus unable to distinguish between a plurality of lenses. In addition, the convex marking in the structure shown in FIG. 5 of the publication projects above the upper flange surface by 0.1 to 0.5 mm. Thus, the lens is unstable when placed with the surface having the convex marking facing down. Further, the convex marking can chip away by contact with another component. The concavity formed at the upper flange surface, on the other side, is likely to suffer from accumulation of contamination, for example glue, which reduces visibility. It is therefore preferred that the marking is visible even if it is formed at the other portion than the flange surface, and it does not touch another component. These references are incorporated herein.
As described earlier, the quality of lenses produced by molding may vary by mold or cavity. This raises the need for quality control, production management, and production tracing of a lens after mounted on an optical device. It is thus preferred to create a marking on a lens itself to identify a production jig such as a mold and a cavity or a production line process. The creation of the marking, however, should not deteriorate the performance of the lens. The lens is an optical component, and certain performance is required accordingly. Creating the marking on an optical functional part at the center of the lens, for example, should be avoided since this has an adverse effect on the optical function.
Further, if a marking is broken by abrasion with another component, it not only reduces visibility of the marking, but also deteriorates the optical performance of the optical functional part due to a broken dust attached thereto.